So I have a 72’ DT2 with aftermarket expansion chamber, mild port job, and Vape electronic ignition set to 2.0mm BTDC.

 I use it for motocross mostly, sometimes a vintage hare scramble, and occasional trail riding. I’m still running the stock carburetor VM26 with stock jetting 30p 160m, needle clip is set to leanest position.

My complaint is that when I use this bike for trail riding it’s breaking up terribly and doesn’t find clean crispy power until I’m beyond 1/4 throttle open. The problem area seems to be right where you rest your throttle position at barely cracked open for any steady RPM cruising. It either wants to be accelerating or decelerating, really comes to life once the needle comes out of the jet. If I richen the needle clip position it behaves worse. I’ve tried leaning out my pilot to a 27.5 and it doesn’t seem to make much of a difference. 

I’m curious if this kind of tuning issue is more to do with needle taper, or slide cut away? It’s a great bike for MX as I’m usually opening the throttle dramatically, but trail and hare scramble it doesn’t love the riding condition and I’m chopping throttle to find power beyond the bottom break up zone. 

I’ve been told by many that the stock 26 is undersized especially for a mildly modded bike, though my understanding is smaller Venturi means more torque, bigger diameter carbs may find me more top end and lose power down low. I’ve seen people talk about VM28-49 spigot style upgrades, or a PWK28 as another option. Just curious as to what some of you might speculate, I’d love to smooth out this trouble area for cleaner response all the way through throttle positions. Also wonder if this bike is just one that wants to be “on the pipe” now that it’s an aftermarket chamber 

You might want to replace the o-ring and retest. Lots vacuum, air screw lean?, needle lean, and still rich? Maybe drawing from bowl when starts to get on needle instead of main jet.

Cutaway effects just off idle

I replaced it a couple of years ago when I restored the bike, you think it could be failing and flooding the bike a bit prematurely and it only recovers once you give the engine more air when the needle and slide lift up out of the hole? Good idea, worth trying at least. Will report back about this one. Would love to eliminate that feature entirely with a different carb anyways

After the mods, you complain that the carb is out of tune.

Common.

 

Merely trying to make it perfect in all ranges. Before the mods it had no top end power at all, I’d take it the way it is before going back to stock. Just looking for some carb tuning insight, is that common?

Very common. Almost required. Look for a 2 stroke tuners guide. It will help guide you, help you understand if it’s too lean (piston frying) or to rich (drowning in fuel).

I'm not up to date on this one so must read back. Just asking are you accidentally running with the choke on as that'll sure make it come up rich from a closed throttle.

There's a "two stroke tuners handbook" in the tech library.